1. Field of the Invention
The present invention relates generally to papermaking, and relates more specifically to multilayer fabrics employed in papermaking. The invention also relates to the binding of triple layer forming fabrics.
2. Discussion of Background Information
In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper “stock”) is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rolls. The belt, often referred to as a “forming fabric,” provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity or vacuum located on the lower surface of the upper run (i.e., the “machine side”) of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, where it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a “press felt.” Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a “batt” layer of the press felt. The paper is then transferred to a dryer section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Typically, papermaker's fabrics are manufactured as endless belts by one of two basic weaving techniques. In the first of these techniques, fabrics are flat woven by a flat weaving process, with their ends being joined to form an endless belt by any one of a number of well-known joining methods, such as dismantling and reweaving the ends together (commonly known as splicing), or sewing on a pin-seamable flap or a special foldback on each end, then reweaving these into pin-seamable loops. A number of auto-joining machines are available, which for certain fabrics may be used to automate at least part of the joining process. In a flat woven papermaker's fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction.
In the second basic weaving technique, fabrics are woven directly in the form of a continuous belt with an endless weaving process. In the endless weaving process, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. Both weaving methods described hereinabove are well known in the art, and the term “endless belt” as used herein refers to belts made by either method.
Effective sheet and fiber support are important considerations in papermaking, especially for the forming section of the papermaking machine, where the wet web is initially formed. Additionally, the forming fabrics should exhibit good stability when they are run at high speeds on the papermaking machines, and preferably are highly permeable to reduce the amount of water retained in the web when it is transferred to the press section of the paper machine. In both tissue and fine paper applications (i.e., paper for use in quality printing, carbonizing, cigarettes, electrical condensers, and like) the papermaking surface comprises a very finely woven or fine wire mesh structure.
Typically, finely woven fabrics such as those used in fine paper and tissue applications include at least some relatively small diameter machine direction or cross machine direction yarns. Regrettably, however, such yarns tend to be delicate, leading to a short surface life for the fabric. Moreover, the use of smaller yarns can also adversely affect the mechanical stability of the fabric (especially in terms of skew resistance, narrowing propensity and stiffness), which may negatively impact both the service life and the performance of the fabric.
To combat these problems associated with fine weave fabrics, multi-layer forming fabrics have been developed with fine-mesh yarns on the paper forming surface to facilitate paper formation and coarser-mesh yarns on the machine contact side to provide strength, stability and life potential. For example, fabrics have been constructed which employ one set of machine direction yarns which interweave with two sets of cross machine direction yarns to form a fabric having a fine paper forming surface and a more durable machine side surface. These fabrics form part of a class of fabrics which are generally referred to as “double layer” fabrics. Similarly, fabrics have been constructed which include two sets of machine direction yarns and two sets of cross machine direction yarns that form a fine mesh paperside fabric layer and a separate, coarser machine side fabric layer. In these fabrics, which are part of a class of fabrics generally referred to as “triple layer” fabrics, the two fabric layers are typically bound together by separate stitching yarns. However, they may also be bound together using yarns from one or more of the sets of bottom and top cross machine direction and machine direction yarns. As double and triple layer fabrics include additional sets of yarn as compared to single layer fabrics, these fabrics typically have a higher “caliper” (i.e., they are thicker) than comparable single layer fabrics. An illustrative double layer fabric is shown in U.S. Pat. No. 4,423,755 to Thompson, and illustrative triple layer fabrics are shown in U.S. Pat. No. 4,501,303 to Osterberg, U.S. Pat. No. 5,152,326 to Vohringer, U.S. Pat. No. 5,437,315 to Ward and U.S. Pat. No. 5,967,195 to Ward. Warp-stitched multilayer fabrics are known in the art. Examples of such fabrics are shown in U.S. Pat. No. 5,152,326 to Vohringer, U.S. Pat. No. 6,202,705 B1 to Johnson and PCT Patent No. WO 02/00996 A1.
U.S. Pat. No. 7,059,357 to WARD, the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses a warp-bound triple layer forming fabric whereby the top yarn parts are vertically stacked over bottom yarns. The warp-stitched fabric is a multilayer papermaker's fabric that has a set of bottom warp yarns, a set of bottom weft yarns, a set of top weft yarns and a set of warp stitching yarn pairs. The bottom warp yarns are interwoven with the bottom weft yarns. The stitching warp yarns interweave with both the bottom weft yarns and the top weft yarns, and are woven such that at locations where the first of the stitching warp yarns in a pair weaves in the top fabric layer, the second stitching warp yarn in the pair drops below the top fabric layer to interweave with one or more bottom weft yarns to bind the top fabric layer and the bottom fabric layer together. The first stitching warp yarn of the stitching warp yarn pair may weave on a first side of one of the bottom warp yarns while the second stitching warp yarn of each stitching yarn pair may weave on the other side of that bottom warp yarn. Each stitching warp yarn pair may be substantially stacked above a bottom warp yarn. The fabric may further include a set of top warp yarns that interweave with the top weft yarns in the top fabric layer. The set of top warp yarns may be woven from a first warp beam, the set of bottom warp yarns may be woven from a second warp beam and the set of stitching warp yarns may be woven from a third warp beam. In WARD, however, only the top fabric is integrally woven. Furthermore, in WARD binding occurs only on the bottom fabric.
U.S. Pat. No. 6,860,299 to KUJI, the disclosure of which is hereby expressly incorporated by reference in its entirety, discloses an industrial multilayer textile that has at least an upper surface side layer and a lower surface side layer as a running surface. The upper surface side layer and the lower surface side layer are connected by warp ground yarn connecting yarns that weave the upper surface side layer and the lower surface side layer. Among these, a pair of warp ground yarn connecting yarns is made into yarns corresponding to one warp in an upper surface side surface, and the pair of warp ground yarn connecting yarns and another warp are alternately disposed and woven with upper surface side wefts to form a surface of a substantially plain weave texture on the upper surface side layer. However, in KUJI the locations where the pairs of crossing warps weave on the bottom are all on one side of a given bottom warp.
The use of zig-zaging binder yarns is also known. However, such a binder arrangement is used only in the context of full warp exchange fabrics whereby all warps weave on the top and the bottom alternately.